1. Field of the Invention
The present invention relates to a thickness-reduced deep-draw-formed can prepared from a resin-coated surface-treated steel plate. More particularly, the present invention relates to a thickness-reduced deep-draw-formed can having a high pressure-resistant vessel strength, an excellent appearance, a high uniformity of the can plate thickness, a good coating adhesion and a high corrosion resistance in combination.
2. Description of the Related Art
As the conventional process for forming a side-seamless can, there has been known a process comprising forming a metal blank such as an aluminum plate, a tinplate or a tin-free steel plate into a cup having a barrel having no seam on the side face and a bottom integrally connected seamlessly to the barrel by subjecting the metal blank to drawing of at least one stage between a drawing die and a punch, and if desired ironing the barrel of the cup between an ironing punch and an ironing die to reduce the thickness of the barrel. It has also been known that in preparing this side-seamless can, a metal plate laminated with a film of a thermoplastic resin such as polypropylene or a thermoplastic polyester is used as the metal blank.
In Japanese Unexamined Patent Publication No. 01-258822, we have proposed a process for reducing the thickness of a side wall of a can by bending and elongation in the above-mentioned deep-draw-forming process, that is, a redrawing process comprising holding a preliminarily drawn cup of a coated metal plate by an annular holding member inserted into the cup and a redrawing die, and relatively moving the redrawing die and a redrawing punch arranged coaxially with the holding member and redrawing die in such a manner that the redrawing punch can come into the holding member and come out therefrom, so that the redrawing punch and redrawing die are meshed with each other, to thereby draw-form the preliminarily drawn cup into a deep-drawn cup having a diameter smaller than that of the preliminarily drawn cup, wherein the curvature radius (Rd) of the working portion of the redrawing die is 1 to 2.9 times the blank thickness (tB) of the metal plate and the curvature radius (Rd) of the holding corner portion of the holding member is 4.1 to 12 times the blank thickness (tB) of the metal plate, flat portions, engaging with the preliminarily drawn cup, of the holding member and redrawing die have a dynamic friction coefficient of 0.001 to 0.2, and draw-forming of at least one stage is carried out so that the redraw ratio, defined by the ratio of the diameter of the shallow-draw-formed cup to the diameter of the deep-draw-formed cup, is in the range of from 1.1 to 1.5, whereby the side wall of the cup is bent and thinned uniformly along the entire height thereof. Furthermore, use of a tin-free steel plate (electrolytically chromate-treated steel plate) coated with an epoxy paint as the coated metal plate has been proposed.
In the draw-redraw forming, plastic flow is caused so that the size of the coated metal plate is increased in the can height direction and is diminished in the circumferential direction of the can barrel. Accordingly, in the can barrel obtained by the draw-redraw forming, the thickness of the side wall tends to increase from the lower portion to the upper portion. In the above-mentioned conventional process for reducing the thickness by bending and elongation, an advantage is attained in that the thickness distribution is uniformalized in the vertical direction, but it has been found that if a surface-treated steel plate as heretofore used for the production of a draw-redraw-formed can is used in this process, problems arise with respect to the pressure-resistant vessel strength, the uniformity of the can plate thickness, the adhesion of the coating and the corrosion resistance.
From the viewpoint of the draw-formability, a cold-rolled steel plate having a high elongation, that is, a low-carbon steel plate, has heretofore been widely used as the surface-treated steel plate for a draw-redraw-formed can. However, in the case where a low-carbon steel plate is used for the production of a thickness-reduced draw-formed can, since the strength of the steel plate is low and the thickness of the side wall is reduced by bending and elongation, the pressure-resistant strength is insufficient. Furthermore, if a steel material having a high elongation is bent and elongated, conspicuous local elongation is readily caused, and by this local elongation, the vessel thickness is rendered uneven and such troubles as formation of pinholes, cracking and peeling are caused in the organic resin coating. As the result, reduction of the adhesion of the coating and exposure of the metal are caused, and the corrosion resistance of the can is drastically degraded.